There is a need for improved Homeland Security Sensing systems that can sense the presence of radiological sources such as those that could be used as dirty bombs (radioactive material to be dispersed by conventional explosives) and the presence of actual nuclear weapons that might be smuggled into the country. Fiber optic sensors have been developed and deployed for a number of sensing applications, and could potentially be useful for sensing radiation sources.
In the past, single mode optical fibers (i.e. conventional fibers) were completely based on index-guiding of light due to doping of glass core to make it have a higher index than the surrounding glass cladding. This produced fibers that typically had a core of 5-10 microns, and a cladding of typically 80-125 microns (FIG. 1), with light mode being confined substantially to the core region. There is typically a plastic jacket around the cladding to protect the glass member, typically of thickness 20-60 microns. One way of employing fiber to make a highly sensitive sensor is to configure the single mode optical fiber in a resonator device. A resonator of this type is highly sensitive to small changes in the loss of the fiber because the light travels through the fiber many times. The issue with using conventional fiber for highly-sensitive radiological sensing is that the core region, where the light is resident, occupies a very small region, namely the core. The fundamental sensitivity of a resonator device is given by the shot noise limit, which improves with the amount of optical power passing through the fiber. However, the optical power in a small-core fiber cannot be arbitrarily increased, since non-linear effects due to high power density ruin the signal to noise. One of these limiting effects is stimulated Brillioun scattering. This effect, for instance, produces instabilities in the signal wave while producing light at another frequency. Thus the primary issue is that small mode-field fibers have low thresholds for Stimulated Brillioun Scattering (SBS), placing optical power limitations on use of these fibers particularly in high signal-to-noise resonator systems. This limits the signal to noise that one can obtain, and thus the minimum resolvable amount radiation.
Optical fiber consists of transparent material such as glass or plastic. Most optical fiber is fused silica and most plastic fiber is polymethylmethacrylate (PMMA). The fiber structure guides light by the process of total internal reflection (TIR). In silica fibers the core is usually established through doping with Germanium. Fibers fall into two basic types, single mode or multimode. In single-mode fibers the core is very small, 5 to 10 microns in diameter, for instance. Multimode fibers have cores of 50 to several thousand microns and very small cladding (in the order of tens of microns). Single-mode fibers have a large cladding (usually more than 50 microns) making the fiber diameter generally 125 microns or more (FIG. 1). The purpose of the large cladding in single-mode fibers is to protect and contain the evanescent field of the single-mode which extends into the cladding for a few microns and can contain more than 10 percent of the optical energy normally thought of as traveling only through the core. Another importance of this larger diameter cladding is so that the fibers can be handled without breaking. With regard to fiber optics used for radiological sensing, radiation from radioactive sources easily penetrates the core and the cladding. However, the dopants inside the core may be chosen to be very sensitive to radiation. These dopants may suffer radiation damage, such as ionization or change atomic state. In turn, this causes loss for light traveling in the fiber, which can be sensed. However, conventional fibers have limitations for signal light power that can be used, and therefore, a larger core is desirable.
Conventional large core fibers (FIG. 2) typically are not single-mode fibers. They propagate a relatively large number of light waves with different spatial distributions, i.e. different spatial modes, possibly in the hundreds. Light traveling in different spatial modes travels at different speeds. Due to unavoidable perturbations, light can and does couple from one mode to another (so-called “mode mixing”). Mode mixing and different light speeds between various modes causes noise and uncertainty in light detection systems and causes pulse spreading in communication systems. For this reason, single-spatial mode (single mode) fibers are used in many communications and sensing systems. While one advantage, of multi-mode fiber is its large core area, the presence of multiple modes and mode mixing renders it unusable in high sensitivity resonator-type sensing devices. What is desirable in the resonator case is a single mode fiber with a large core area.
Resonators have been proposed for use in radiological sensors to circulate light around an optical fiber loop for multiple passes. A periodic series of resonance lineshapes is produced, each having a peak centered about a resonance frequency under normal conditions, and the resonance lineshape has a finesse associated therewith. The frequency-periodicity of frequency separation between resonance frequencies of the same mode is the free spectral range of the resonator. As used herein, the term “finesse” refers to a relationship (e.g., sharpness) based on a ratio of the free-spectral range to the linewidth of an individual resonance lineshape. The linewidth of the resonance lineshape is a frequency width at half of the maximum peak value of the resonance lineshape. The finesse additionally relates to the number of times the light recirculates within the optical loop with reproducibility, and thus is inherently related to the round-trip loss of the resonator. Higher losses generally result in lower finesses. Changes in the finesse, and the amount of light circulating within the resonator can be measured as an indication of changes to the resonator fiber loss, and therefore, to exposure to loss-inducing radiation. The signal to noise of the measurement is determined by the power circulating in the resonator provided there is no significant spurious noise from other modes. It is generally difficult to couple light into a multi-mode optical fiber and maintain the light in a single spatial mode that reproduces itself for multiple circulations through the resonator. For example, perturbations (e.g., imperfections, geometrical distortions, etc.) along the length of the optical fiber typically decrease the round-trip reproducibility of the single fiber spatial mode within a multi-mode fiber, and thus decrease the finesse. Other spatial mode resonances can also be excited which typically cause errors in the intended measurement. In the latter case, a complex structure of resonances, which may be based on a single stable resonance, may be observed that create instabilities and errors in the measurement. Each spatial mode may be associated with two polarization modes, which doubles the number of resonances in the spectrum.